A wooden house generally has a wall structure in which an external wall material forming an outdoor side wall of the house and an internal wall material forming an indoor side wall of the house are fixed to columns erected on a groundsill, and a heat insulating material, a waterproof sheet, etc. are disposed in a space between the external wall material and the internal wall material. The surface of the external wall material is finished, for example, in siding or mortar, and the surface of the internal wall material is finished, for example, with cloth hanging or the like.
With such a conventional construction method for a wooden house, all work for constructing walls, such as the work of fixing the external wall material and the internal wall material to the columns, and the work of filling the heat insulating material into the interior of the wall, are performed manually by carpenters or different builders at the construction site. Thus, the problems arise that the construction period is long, and accordingly the construction expenses are high.
In the house formed by the above construction method, problems, such that dew condensation causes fungi in the interior of the wall (the space between the external wall material and the internal wall material), or corrodes the column or the wall material, are likely to occur.
To solve such problems, a wall panel (building material) comprising the above-mentioned external wall material, internal wall material and heat insulating material integrated has found use in recent years. Various structures have been proposed for wall panels and, among them, there is, for example, a structure in which a high performance material is integrally provided between a first plate material and a second plate material, each made of Paulownia tomentosa (paulownia) (see Patent Document 1).
[Prior Technical Literature]
[Patent Document]
    [Patent Document 1] JP-A-2008-14111